1. Field of the Invention
The present invention relates to an injection control system of a diesel engine for performing an injection quantity learning operation.
2. Description of Related Art
As a method of inhibiting combustion noise or generation of nitrogen oxides in a diesel engine, a method of performing a pilot injection for injecting a very small quantity of fuel before a main injection is known. Since a command value of the pilot injection quantity is small, improvement of injection accuracy is necessary to sufficiently exert the effect of the pilot injection of inhibiting the combustion noise and the generation of the nitrogen oxides. Therefore, an injection quantity learning operation for measuring a deviation between the command injection quantity of the pilot injection and a quantity of actually injected fuel (an actual injection quantity) and for correcting the injection quantity on a software side is necessary.
A fuel injection control system described in Japanese Patent Application No. 2003-185633 can perform the injection quantity learning operation highly accurately. The control system controls an injection pressure (a pressure of fuel in a common rail) to a target injection pressure for the learning operation while an operating state of the engine is in a deceleration-and-fuel-cutting state (a state in which fuel supply is cut and a vehicle is decelerated). Then, the control system performs a single injection for the learning operation from an injector into a specific cylinder. The control system learns (corrects) the injection quantity based on a fluctuation of an engine rotation speed caused by the single injection.
Timing for performing the single injection is an important factor to realize highly accurate correction in the injection quantity learning operation. More specifically, if the timing of the single injection is too early, there is a possibility that a condition suitable for measuring the rotation speed fluctuation caused by the single injection has not been established yet. For instance, if the single injection is performed at too early timing at which a load of the fuel pump is still unstable and is causing a rotation speed fluctuation, a learned value of the injection quantity will include an error. If the timing of the single injection is too late, a period necessary to accomplish the learning operation is lengthened. In this case, there is a possibility that the learning condition is broken if the injection is resumed when a vehicle driver accelerates a vehicle again or if the injection is resumed to prevent engine stall when the rotation speed decreases to proximity of an idling rotation speed, for instance. In such a case, the learning operation cannot be completed. Thus, determination of suitable single injection timing is important.
As explained above, in this injection quantity learning operation, the step of decelerating the vehicle and cutting the fuel supply, the step of controlling (increasing or decreasing) the injection pressure to the target injection pressure, the step of injecting the fuel into the specific cylinder, and the step of measuring the rotation speed fluctuation caused by the single injection are performed in that order. The engine drives the fuel pump. Accordingly, if the load of the fuel pump increases, or if a quantity of the fuel pressure-fed by the fuel pump increases, the load of the fuel pump affects the engine rotation speed (for instance, the engine rotation speed decreases). Moreover, the load of the fuel pump affects the rotation speed fluctuation caused by the single injection. Therefore, the single injection is performed in the specific cylinder under a condition that the injection pressure is controlled to the target injection pressure and the rotation speed fluctuation caused by the pump load fluctuation due to the injection pressure control subsides. It is required that the load of the fuel pump should be stable (or the load of the fuel pump should not fluctuate greatly) while the rotation speed fluctuation caused by the single injection is measured.
The load of the fuel pump is associated with a fuel pressure-feeding quantity of the fuel pump. An electronic control unit (ECU) determines the fuel pressure-feeding quantity based on at least the target injection pressure and the present injection pressure. Therefore, the load of the fuel pump can be estimated based on a command pressure-feeding quantity outputted to the fuel pump. It can be determined that the load of the fuel pump is stabilized if the command pressure-feeding quantity outputted to the fuel pump does not fluctuate for a predetermined period. However, in this case, there is a possibility that the single injection timing is delayed.